Different integrated circuits (ICs), which perform different functions, are, in many cases, constructed from individual transistors which are themselves quite similar. Gate array technology takes advantage of this similarity, by using identical, prefabricated arrays of transistors as the starting point for fabrication of the ICs.
For example, a manufacturer of ICs will stockpile, in advance, multiple copies of several different types of transistor arrays. Then, when the manufacturer receives an order for a particular IC, the manufacturer will select the best array, and complete the fabrication steps for the IC, using the selected array.
This approach reduces the processing time between the receipt of the order and the shipping of the completed IC, because many of the basic fabrication steps have been completed in advance.
Typical gate arrays are constructed of digital, as opposed to analog, transistors. The two types perform different functions. The digital type acts as a switch: it is either ON or OFF. The analog type acts as an amplifier: it amplifies a signal.
These two types are constructed differently: Analog transistors should have long channels with large surface area: their channels should be long and wide.
The channel must be long, in order to obtain high gain. [See, for example, S. M. Sze, Physics of Semiconductor Devices, (1969, John Wiley, ISBN 471 84290 7) chapter 7, section 3, p. 340 et seq].
The channel must be wide, in order to reduce a particular type of electrical noise, namely, "l/f noise." This type of noise is so-called because it has been experimentally found to be nearly inversely proportional to frequency, as the designation "l/f" indicates. Because of the inverse proportionality, much of the noise power is concentrated at low frequencies. If gain at low frequencies is important, then the "l/f noise" presents a problem. l/f noise is reduced by increasing surface area of the channel.
Digital transistors should have channels as short as possible. However, since a short channel produces a large electric field along the channel's length (i.e., between the source and the drain), the channel should be sufficiently long that the electric field does not exceed the breakdown value of the channel material.